docs: def flatten(items):
      """
      Yield items from any nested iterable. Used by ``QuadTree.flatten`` to one-dimensionalize a list of sublists.
      cf. http://stackoverflow.com/questions/952914/making-a-flat-list-out-of-list-of-lists-in-python
      """
      for x in items:
          if isinstance(x, Iterable):
              yield from flatten(x)
          else:
              yield x